aarch64: Add a separate "SVE sizeless type" attribute

aarch64: Add a separate "SVE sizeless type" attribute

It's more convenient for a later patch if sizelessness is represented
separately from "SVEness".  "SVEness" is an ABI property that carries
forward into gimple and beyond, and continues to matter during LTO.
Sizelessness is just a frontend restriction and can be ignored after
that.

2020-04-09  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* config/aarch64/aarch64.c (aarch64_attribute_table): Add
	"SVE sizeless type".
	* config/aarch64/aarch64-sve-builtins.cc (make_type_sizeless)
	(sizeless_type_p): New functions.
	(register_builtin_types): Apply make_type_sizeless to the type.
	(register_tuple_type): Likewise.
	(verify_type_context): Use sizeless_type_p instead of builin_type_p.